Hmmmm I know the problem you are talking about Nick, but I'm not aware of a fix from Techspeed. If a fix were indeed possible, why haven't MGR done anything yet? This 'design characteristic' has been with this gearbox since inception.
The PG1 gearbox fitted to MG ZR/ZS/25/45 models now has a synchro device on reverse gear (which the R65(U)) gearbox lacks.
Partial engagement of 4th gear when selecting reverse gear, slows/synchronises the various shafts/gears and provides a crunch free change....It works too!
It might be worth checking the clutch is not dragging - this would aggrevate the problem.